HE-ISOTOPE SYSTEMATICS OF SIBERIAN DEPLETED MANTLE PERIDOTITES: A PETROLOGIC PERSPECTIVE

He-isotope systematics have been widely used to constrain the structure and chemical evolution of the mantle (e.g., MORB = 8 ± 1 R_A). However, the sub-continental lithospheric mantle (SCLM) remains poorly characterized, with the few existing studies investigating mostly metasomatized spinel-facies peridotites (Gautheron et al., Chem. Geol, 2005; Tang et al., J Asian Earth Sci, 2014). We report new He-isotope data on a suite of olivine mineral-separates from well characterized garnet-bearing peridotites; including one lherzolite, one harzburgite, and 10 dunite xenoliths from the Udachnaya kimberlite pipe, Siberia. Olivine He-isotope data from other lithospheric regions display a limited range (e.g., 6.1 ± 0.9 R_A, Gautheron and Moriera, EPSL, 2002). This has been argued to result from a steady-state flux of ³He through the SCLM, associated with metasomatic fluids derived from the asthenosphere. However, our data, in combination with olivine data one other refractory peridotite from Udachnaya (Barry et al., AGU Fall Meeting, 2013), display a much larger range in He-isotope ratios (2.3-9.8 R_A). This suggests that He-fluxes within the SCLM are significantly more heterogeneous than previously thought. Importantly, this suite is characterized by a high Fo content (> 92), typical of a ‘pristine’ melt residue. The un-metasomatized nature of Udachnaya dunites suggests that they have been largely isolated from metasomatic fluxes over geological time. This is potentially due to their position within the lithosphere (i.e., shallow spinel-facies peridotites versus deep garnet-facies peridotites) and/or lithospheric age. The oldest (Archean) samples display the highest He-isotopes, suggesting that ancient craton stabilization may be linked to the extraction of large volumes of komatiitic magmas from a plume source. In contrast, younger (Proterozoic) samples are characterized by He-isotopes below the MORB range, potentially reflecting high U-Th additions to the lithosphere by the accretion of ancient island-arc terrains at this time.